Because mostly tariffs are advertised with an “up to” value. Or you just want to check whether your provider has provided the correct information for the tariff (s) booked. Perhaps you also want to compare tariffs and see what service you are currently satisfied with. Perhaps someone advised you to do a speed test and you are now wondering why you should check the DSL or cable connection. The latter should of course only be switched off shortly before the test and on again immediately after the test for optimal protection.
In addition to mail programs, Dropbox, web browsers (or other tabs in it), iTunes and Co., this also includes Messenger, the antivirus program and so on.
Sir Apfelot recommendation: Clean up your Mac hard drive with CleanMyMac How fast is my internet?